404 THE INDUCTIONS OF BIOLOGY. 



accessible habitat, eacli species spreads until it reaches limits 

 which are for the time insurmountable. 



§ 107. We pass now to the distribution of organic forms in 

 Time. Geological inquiry has established the truth that 

 during a Past of immeasurable duration, plants and animals 

 have existed on the Earth. In all countries their buried 

 remains are found in greater or less abundance. From com- 

 paratively small areas multitudinous diljerent types have been 

 exhumed. Every exploration of new areas, and every closer 

 inspection of areas already explored, brings more types to 

 light. And beyond question, an exhaustive examination of 

 all exposed strata, and of all strata now covered by the sea, 

 would disclose types immensely out-numbering those at 

 present known. Further, geologists agree that even had we 

 before us every kind of fossil which exists, we should still 

 have nothing like a complete index to the past inhabitants of 

 our globe. Many sedimentary deposits have been so altered 

 by the heat of adjacent molten matter, as greatly to obscure the 

 organic remains contained in them. The extensive formations 

 once called " transition," and now re-named " metamorphic," 

 are acknowledged to be formations of sedimentary origin, from 

 which all traces of such fossils as they probably included have 

 been obliterated by igneous action. And the accepted con- 

 clusion is that igneous rock has everywhere resulted from 

 the melting-up of beds of detritus originally deposited by 

 water. How long the reactions of the Earth's molten nucleus 

 on its cooling crust, have been thus destroying the records of 

 Life, it is impossible to say; but there are strong reasons for 

 believing that the records which remain bear but a small ratio 

 to the records which have been destroyed. Thus we have 

 but extremely imperfect data for conclusions respecting the 

 distribution of organic forms in Time. Some few generaliza- 

 tions, however, may be regarded as established. 



One is that the plants and animals now existing mostly 

 differ from the plants and animals which have existed. 



